Python: What is the descriptors protocol?

1class Validator:
2    def __init__(self, min_value=None, max_value=None):
3        self.min_value = min_value
4        self.max_value = max_value
5
6    def __set_name__(self, owner, name):
7        self.name = name
8
9    def __get__(self, obj, objtype=None):
10        if obj is None:
11            return self
12        return obj.__dict__.get(self.name)
13
14    def __set__(self, obj, value):
15        if self.min_value is not None and value < self.min_value:
16            raise ValueError(f"{self.name} must be >= {self.min_value}")
17        if self.max_value is not None and value > self.max_value:
18            raise ValueError(f"{self.name} must be <= {self.max_value}")
19        obj.__dict__[self.name] = value
20
21class Student:
22    age = Validator(min_value=0, max_value=150)
23    grade = Validator(min_value=1, max_value=12)
24
25s = Student()
26s.age = 25  # OK
27# s.age = -5  # ValueError!